Spatial mismatch between phytoplankton and zooplankton biomass at the Celtic Boundary Front

Abstract

Thermohaline fronts are present globally and have been identified as biodiversity hotspots, encouraging enhanced primary productivity and attracting dense aggregations of higher trophic level organisms. The Celtic Boundary Front (CBF) sits in an important zone of economic activity yet no contemporary studies describing the meso-zooplankton community exist for this region. High-frequency sampling during June 2009 revealed three distinct areas on the mixed (Irish Sea) and stratified (Celtic Sea; Celtic Deep) sides of the front, each exhibiting distinct physical and biological characteristics. Low zooplankton abundance was found above the Celtic Deep, despite apparent optimal phytoplankton conditions, conflicting with observations made at other frontal locations. Although zooplankton assemblages were generally distinct within areas, the copepod Acartia clausii was ubiquitous across all three areas and accounted for almost 60% of all counted individuals. The long-term variation of zooplankton during the seasonal front development was examined using samples from the Continuous Plankton Recorder. The results of this analysis suggest that low abundance is a recurring feature over the Celtic Deep. Possible explanations for the observed patterns may be top-down effects on zooplankton from pelagic fish; alternatively, these patterns may be a consequence of the strong density and stratification gradients in the Celtic Deep.